Antenna



Dec. 7, 1948. G. l-LBROWN 2,455,403

ANTENNA Filed Jan. v2O, 1945 Patented Dec. 7, 1948 ANTENNA George H.Brown, Princeton, N. J., assigner to Radio Corporation of America, acorporation of Delaware Application January 20, 1945, Serial No. 573,724

4 Claims.

This invention relates to antennas, and more particularly to broad-banddirective antennas suitable for directional transmission oi televisionsignals and the like.

The principal object of the instant invention is to provide an improvedmethod of and means for compensating the variations with frequency ofthe reactances of the radiator elements of a directive antenna.

Another object is to provide an improved directive antenna in which thereactance of one radiator or group of radiators is balanced by thesimilar reactance of another radiator or group of radiators, throughouta relatively wide band of frequencies.

A further object of the present invention is to provide an antennasystem of the described type which is simple and rugged in structure,easily designed for specific performance requirements, and involves nocritical adjustments.

The invention will be described with reference to accompanying drawing,oi which Fig-ure 1 is a schematic plan view of an antenna systemembodying the invention,

Figure 2 is a geometrical diagram used in deriving the directive patternof the system of Figure l,

Figure 3 is a graph showing the directive pattern of an antenna of thetype illustrated in Figure l, as calculated and as measuredexperimentally, and

Figure 4 is a schematic diagram oi a modified form oi the antenna ofFigure 1.

Referring to Figure l, a pair oi dipole structures I and 3 are supportedin front of a conductive screen or reflector 5. Side members 'I, similarto :1,

radiators I and 3 may be oi any known type. In the present example, eachradiator comprises a so-called slotted dipole, including cylindricalradiator elements I! and I3 secured to a tubular support i5, which isprovided with a slot .il extending approximately one-quarter waveilength down from the elements Il and I3. The radiator I' is fed by acoaxial line I9, which has its outer conductor connected to the supportI5 and its inner conductor extending up within the slotted portion ofthe support I5 and connected to the junction of the radiator element IIwith the support I5. This arrangement constitutes a simple and effectiveway oi feeding the dipole, which is symmetrical or balanced to ground,from the coaxial line I9, which is unsymmetrical to ground.

The radiator 3 is connected like the radiator I to a coaxial line 2l.The lines I9 and 2I diier in length by one-quarter wavelength, the lineI9 in this case being one wavelength long, while the line 2I is 3Awavelength long. The lines I9 and 2I are connected together at ajunction point 23, to which a main feed line 25 is also connected. Thefinal portion of the main feed line 25 may include an impedancetransformer of the cascaded quarter wave line type, such as thatdescribed and claimed in U. S. Patent 2,249,597. In the presentillustration the transformer includes two sections 2'I and 29. Thesection 21 is designed to have a characteristic impedance somewhathigher than the impedance presented at the junction point 23 by thelines I9 and 2i and the section 29 is designed to have a characteristicimpedance somewhat lower than that of the line This arrangement enablesthe use of identical structures for the lines I9, 2 I, and 25, Whilemaintaining impedance match throughout a broad band of frequencies. Thedipoles I' and 3 are preferably designed to match the lines I9 and 2I attheir resonant frequency.

In the operation of the above-described system, energy applied to thejunction point 23 through the feed line 25 is divided equally betweenthe radiators I and 3. At the resonant frequency, the radiators presentpurely resistive impedance to their respective lines I9 and 2l, so thatno standing waves appear in the system. At frequencies lower than theresonant frequency, the impedances presented by the dipoles includecapacitive reactance. The reactance of the radiator I is transferredwithout change by the line I9 to the point 23, where it appears as ashunt capacitance. Since the line 2I is only 2%; wavelength long,however, the reactance of the radiator 3 is inverted, appearing at thepoint 23 as a shunt inductance. The inductive reactance presented by theline ZI is equal to the capacitive reactance presented by the line I9,so that the net effect at the point 23 is that of a parallel resonantcircuit of such high impedance as to have a practically negligibleshunting eiect. Thus only the resistive components of the impedances ofthe radiators I and 3 remain at the point 23 and the impedance match tothe feed line 25 is substantially the same as at resonance. Operation atfrequencies higher than the resonant frequency is similar to thatdescribed above with the exception that both radiators are inductive, sothat the line 2l presents capactive reactance at the junction 23 Whilethe line i9 presents an equal inductive reactance. This reactancecancellation is effected throughout a very wide band of frequencies,enabling eiicient power transfer over a frequency range of better than2:1.

Since the radiators l and 3 are fed in quadrature, rather than in phase,the directive pattern will be somewhat unsymmetrical, with the axis ofmaximum radiation extending slightly to one side of the mechanicalcenter line of structure. Referring to Figure 2, the radiators arerepresented by points i and 3, disposed equally distant on oppositesides of the axis A--A. In the present instance, it is assumed that theradiators are separated from each other by ZA; wavelength. The dipoles land 3 are fed in quadrature. The theoretical horizontal pattern isproportional to where gb is the angle referred to the axis A-A The firstterm is the directivity function of two point sources spaced 240 and fedin quadrature. The second term is the factor for the image produced byan innite screen 90 behind the radiators. The third term is thedirectivity factor for a short dipole. This directive pattern isrepresented graphically in Figure 3 by the solid curve.

Experimental measurement of the directive pattern of an antenna similarto that of Figure l results in the dash curve or" Figure 3. It isapparent that the direction of maximum radiation is at an angle ofapproximately 20 to the physical axis of the structure. Since thereflector tends to provide concentration of the radiation directly alongthe physical axis, the sharpness of the beam may be improved to a slightextent by displacing the radiators l and 3 with respect to each other inthe direction of the axis, so as to align their pattern. with that ofthe reflector. r)This 4arrange,- ment is illustrated in Figure 4, wherethe radiator l is advanced by a distance S with respect to the radiator3. The perpendicular bisector BWB of the line between the radiators isnow at an. angle of yto the axis A--A, where The distance S is made suchthat the angle a is approximately 20.

Although the invention has been described with reference to a systemincluding only two radiator elements, it will be apparent to thoseskilled in the art that any desired number of such pairs oi radiatorelements may be employed as subcombinations in a complex array.Furthermore, each of the radiator elements of the described system maybe replaced by a plurality of radiator elements conne-cted together.Briefly, the invention contemplates the use of pairs of radiatorsshielded from each other and energized from a common feed point throughtransmission lines which diier in length by 1/4 Wavelength. Thisarrangement results in cancellation of the reactances of the radiatorsat the common feed point.

I claim as my invention:

l. A directive antenna system including a pair of parallel radiatorelements each provided with a plane reflector, a shield between saidradiators who eby direct space coupling between said radiators isavoided, transmission lines coupled respectively to each of saidantennas and to a common feed line, the lengths of said. transmissionlines differing by one quarter of the operating wavelength, one of saidradiator elements and its associated reflector being spaced from theother along a line normai to the plane of said reflectors a distancesuch that the maximum response ci said antenna system is along saidline.

2. A directive antenna system including a nui ber of parallel radiatorelements each provided with a plane reflector, a shield between saidradiators whereby direct coupling,- between said radiators is avoided,transmission lines coupled respectively to each of said antennas and acommon feed line, the lengths oi said transmission lines diifering byone quarter of the operating Wavelength, half oi said radiator el itsand their associated reflectors being spaced irorn the remainder along aline normal to the plane oi said reflectors a distance such that the.maximum response of said antenna system is along said line.

3. A directive antenna system including a pair of parallel radiatorelements each provided with a plane reflector, a shield between saidradiators whereby direct space coupling between said radiators isavoided, said radiator elements being spaced apart a distancesubstantially equal to two thirds oi the operating wavelength, trmission lines coupled respectively to each oi' said antennas and to acommon feed line, the lengths of said transmission lines differing byone cuarter of the operating wavelength, one ci said radiator elementsand its associated reflector being spaced from the other along a linenormal to the plane of said reflectors a distance such that the maximumresponse oi said antenna system is along said line.

4. A directive antenna sys tem including a nurnber of parallel radiate.1elements each provided with a plane reflector, a shield bet een saidradiators whereby direct space coupii between sind radiators is avoided,said radiati elements being spaced apart distance .subs .itially equalto two thirds of the operating Wavelength, transmission lines coupledrespectively to each of said antennas and to a common, ieed line, theleng of said transmission lines differing ley one guar of the operatingwavelength, haii said radiator elements and their associated reiiectcrsbeing spaced from the remainder along a line normal to the plane of saidreflectors a distance such that the maximum response oi antenna systemis along said line.

GEORGE ii. BROWN.

@ETRE The following references are of record in the nie of this patent:

UNITED STATES PATENTS Number Name Date 20,922 Lindenblad Nov. 22, i9382,275,646 Peterson Mai'. l0, 1942 2,380,333 Scheldori July l0, 1945

